The configuration and operation of a conventional ordinary solid polymer electrolyte type fuel cell will be described in connection with FIGS. 1, 2 and 7. FIG. 1 depicts the basic configuration of a polymer electrolyte type fuel cell (hereinafter referred to as “PEFC”) among the conventional fuel cells. A fuel cell is an apparatus to cause electrochemical reaction of a fuel gas such as hydrogen with an oxygen-containing gas such as air on a gas diffusion electrode to generate electricity and heat at the same time. As an electrolyte 1 there is used a polymer electrolyte membrane which selectively transports hydrogen ion or the like. On the both surfaces of the electrolyte 1 is provided a catalytic reaction layer 2 comprising as a main component a carbon powder having a platinum-based metallic catalyst supported thereon in contact therewith. On this catalytic reaction layer, reactions represented by the (chemical formula 1) and (chemical formula 2) occur and, on the whole of the fuel cell, reactions represented by the (chemical formula 3) occurs.H2→2H+2e−  (chemical formula 1)½O2+2H++2e−→H2O  (chemical formula 2)H2+½O2→H2O  (chemical formula 3)
The fuel gas containing at least hydrogen (hereinafter referred to as “anode gas”) undergoes a reaction represented by the (chemical formula 1) (hereinafter referred to as “anode reaction”) while the hydrogen ion which has moved through the electrolyte 1 undergoes a reaction represented by the (chemical formula 2) (hereinafter referred to as “cathode reaction”) with an oxygen-containing gas (hereinafter referred to as “cathode gas”) on catalytic reaction layer 2 to produce water, whereby electricity and heat are generated. On the whole of the fuel cell, hydrogen and oxygen react with each other as represented by the (formula 3) to produce water electricity and heat can be used. The side on which the fuel gas such as hydrogen takes part in the reaction is called anode and is given a sign a in the drawings and the side on which the oxidizing agent gas such as air takes part in the reaction is called cathode and is given a sign c in the drawings. Further, diffusion layers 3a and 3c having both gas permeability and electrical conductivity are disposed in close contact with the outer surface of the catalytic reaction layers 2a and 2c, respectively. The diffusion layer 3a and the catalytic reaction layer 2a form an electrode 4a and the diffusion layer 3c and the catalytic reaction layer 2c form an electrode 4c. The reference numeral 5 indicates an electrode-electrolyte assembly (hereinafter referred to as “MEA”) and is formed by the electrode 4 and the electrolyte 1. A pair of electrically-conductive separators 7a and 7c disposed thereon which mechanically fix MEA 5 and electrically connect adjacent MEAs 5 in serial, and have gas flow paths 6a and 6c formed on the surfaces thereof in contact with MEA 5 for supplying the reactive gas into the electrodes and carrying the gas produced by the reaction or extra gas away is provided. The electrolyte 1, the pair of catalytic reaction layers 2a and 2c, the pair of diffusion layers 3a and 3c, the pair of electrodes 4a and 4c and the pair of separators 7a and 7c form a basic unit of a fuel cell (hereinafter referred to as “cell”). The separators 7a and 7c have the separator 7c and 7a of the adjacent cell disposed in contact therewith, respectively, on the surface thereof opposite MEA 5. The separators 7a and 7c have a cooling water flow path 8 provided on the side thereof in contact with each other through which cooling water 9 flows. The cooling water 9 moves heat such that the temperature of MEA 5 is adjusted through the separators 7a and 7c. MEA 5 and the separator 7a or 7c are sealed to each other with an MEA gasket 10 and the separators 7a and 7c are sealed to each other with a separator gasket 11.
The electrolyte 1 has a fixed charge and as a counter ion for the fixed charge there is present hydrogen ion. The electrolyte 1 is required to be selectively permeable to hydrogen ion, and, to this end, it is necessary that the electrolyte 1 have water content retained therein. This is because when the electrolyte 1 contains water content, the fixed charge fixed in the electrolyte 1 is ionized, causing hydrogen which is a counter ion for the fixed charge to be ionized and movable.
FIG. 2 depicts a stack comprised of laminating cells. Since the voltage of a fuel cell unit is normally as low as about 0.75 v, a plurality of cell units is laminated in series with each other to give a high voltage. A collector 21 is adapted to take current out of the stack and an insulating plate 22 is adapted to electrically insulate the cell from the exterior. An end plate 23 is adapted to fasten and mechanically retain the stack having cell units laminated on each other.
A conventional fuel cell system will be described in connection with FIG. 38. A fuel cell system is received in an outer case 31. A gas purifying portion 32 is adapted to remove materials having adverse effects on the fuel cell from the fuel gas and introduce the fuel gas from the exterior through a raw material gas pipe 33. A valve 34 is adapted to control the flow of raw material gas. A fuel generator 35 is adapted to produce a fuel gas containing at least hydrogen from the raw material gas. The fuel gas is introduced from the fuel generator 35 into the stack 38 through a fuel gas pipe 37. A blower 39 is adapted to introduce an oxidizing agent gas into the stack 38 through a suction pipe 40. The suction pipe 42 is adapted to discharge the oxidizing agent gas discharged from the stack 38 out of the fuel cell system. The fuel gas which has not been used in the stack 38 is allowed to flow again into the fuel generator 35 through an off-gas pipe 48. The gas from the off-gas pipe 48 is used in combustion and is utilized in endothermic reaction for the production of a fuel gas from the raw material gas, etc. An electric power circuit portion 43 is adapted to take electric power out of the fuel cell stack 38 and a control portion 44 is adapted to control the gas and the electric power circuit portion. A pump 45 is adapted to cause water to flow from a cooling water inlet pipe 46 through the water line into the fuel cell stack 38. The water which has flown through the fuel cell stack 38 is carried out through the cooling water outlet pipe 47. Water flows through the fuel cell stack 38; thereby the heat generated in the fuel cell stack 38 can be used outside the fuel cell system while keeping the heated fuel cell stack 38 at a constant temperature. The fuel cell system is formed by the stack 38 composed of fuel cells, the gas purifying portion 32, the fuel generator 35, the electric power circuit portion 43 and the control portion 44.
The household fuel cell system is formed by the fuel cell stack 38 and the fuel generator 35. It is necessary that the deterioration of performance of the fuel cell system be eliminated to maintain desired performance over an extended period of time. Further, in the case where a raw material gas such as city gas comprising methane as a main component is used for household, it is effective to operate the household fuel cell system for a time zone during which electricity and heat are consumed in a large amount while suspending the household fuel cell system for a time zone during which electricity and heat are consumed less for the purpose of enhancing the advantage of fuel and light expenses and the effect of reducing CO2.
In general, DSS (Daily Start & Stop or Daily Start-up & Shut-down) operation in which operation is conducted in the daytime but is suspended in the nighttime can enhance the advantage of fuel and light expenses and the effect of reducing CO2 and the fuel cell system preferably can flexibly cope with an operation pattern comprising starting and suspension. Some reports have been made to date.
For example, as a method of solving these problems, an electric power consuming unit is separately connected to the interior of the system until the starting of connection of an external load to the system during starting to prevent the system from reaching the open circuit potential (see JP-A-5-251101). Alternatively, a discharging unit of inhibiting the open circuit voltage is installed in the system (see JP-A-8-222258). Alternatively, the system is suspended and stored while being enclosed with moistened inert gas to keep an ion exchange membrane as an electrolyte impregnated with water also during storage (see JP-A-6-251788). In order to prevent the oxygen electrode from being oxidized or having impurities adsorbed thereto, electricity generation is conducted while suspending the supply of an oxygen-containing gas so that the consumption of oxygen can be adjusted to enhance the durability (see JP-A-2002-93448). Alternatively, hydrogen leaking from the anode to the cathode is used to enhance the performance of the cathode electrode (see JP-A-2000-260454).
In order that the electricity-generation reaction on the electrode in the aforementioned fuel cell might occur invariably over an extended period of time, it is necessary that the interface of the electrolyte with the electrode be kept stable over an extended period of time. It is said that the open circuit voltage of a polymer electrolyte type fuel cell using hydrogen and oxygen as reaction species is theoretically 1.23 V. However, the actual open circuit voltage indicates the mixed potential of impurities and adsorption species on the various electrodes, i.e., hydrogen electrode and oxygen electrode ranging from about 0.93 V to 1.1 V. Further, some voltage drop due to the diffusion of hydrogen and oxygen in the electrolyte occurs. It is thought that if no extreme dissolution of impurities such as metallic species occurs, the potential of the hydrogen electrode is greatly affected by adsorbing species of the air electrode and is attributed to the mixed potential caused by chemical reactions represented by the (chemical formula 4) to (chemical formula 8) (see H. Wroblowa, et al., “J. Electroanal. Chem., 15, p 139-150 (1967), “Adsorption and Kinetics at Platinum Electrodes in The Presence of Oxygen at Zero Net Current” as reference). Thus, a problem arises that when the voltage exceeds 0.88 V, the oxidation of Pt occurs as represented by the (chemical formula 7), causing not only the deterioration of catalytic activity of PT but also the dissolution and elution of Pt with water.O2+4H++4e−=2H2O 1.23 V  (chemical formula 4)PtO2+2H++2e−=Pt(OH)2 1.11 V  (chemical formula 5)Pt(OH)2+2H++2e−=Pt+2H2O 0.98 V  (chemical formula 6)PtO2+2H++2e−=2H2O+Pt 0.88 V  (chemical formula 7)O2+2H++2e−=H2O2 0.68 V  (chemical formula 8)
Therefore, the conventional technique discloses a method of preventing open circuit but doesn't disclose that the voltage should be not higher than 0.88 V.
Further, the conventional method of purging the anode or cathode with water or moistened inert gas doesn't disclose that the potential of the various electrodes should be kept not higher than a predetermined value and thus is disadvantageous in that when the interior of the cell is filled with an inert gas, both the anode and the cathode cannot be kept at a constant potential and range from about 0.93 V to 1.1 V due to oxygen which gradually enters from the exterior, causing themselves to be oxidized or eluted and hence resulting in the deterioration of performance.
Moreover, the aforementioned conventional method of purging with water or moistened inert gas is disadvantageous in that the temperature of the stack 38 falls during suspension to make dew condensation in the fuel cell stack 38, causing a volume drop resulting in the reduction of pressure therein that then causes the inflow of external oxygen, the damage of the electrolyte 1 or the shortcircuiting of the electrodes 4a and 4c. 
Further, the aforementioned conventional method which comprises causing the cell to generate electricity while suspending the supply of an oxidizing agent gas so that the consumption of oxygen in the gas flow path 6C is followed by the purging of the gas flow path 6a with an inert gas is disadvantageous in that the effect of oxygen left unconsumed in the gas flow path 6c or air which enters due to diffusion or leakage causes the electrode 4c to be oxidized or deteriorated. Moreover, another disadvantage is that since the electricity generation causes forced consumption of oxygen, the potential of the electrode 4c is not uniform and the cathode is activated differently from suspension period to suspension period, causing the cell voltage during starting to vary.
Further, the method which comprises enhancing the performance of the cathode electrode by hydrogen which leaks to the cathode, which has more air present in the anode, has a disadvantage that the mixing of oxygen and hydrogen causes the instabilization of potential resulting in the diffusion of the performance of the cathode.
Moreover, the method which allows hydrogen to flow into the cathode to enhance the performance of the cathode electrode has a disadvantage that the proportion of hydrogen which is not used in electricity generation rises, causing the drop of electricity generating efficiency per energy.